Role of proline-linked pentose phosphate pathway in biosynthesis of plant phenolics for functional food and environmental applications: a review

Phenolic phytochemicals from food-grade plants that are antioxidants are an important part of a healthy diet in a global population that is projected to reach 9 billion in the next 50 years. Such phytochemicals are being targeted for designing conventional foods with added health benefits and are called functional foods. These value-added foods are needed for dietary support to manage major oxidation-linked diseases such as diabetes. cardiovascular diseases, arthritis, cognition diseases and cancer. Plants produce phenolic metabolites during growth and developmental and stress adaptation responses. These phenolic phytochemicals can be targeted for designing functional foods and in order to design consistent food-grade phytochemical profiles for safety and clinical relevancy, novel tissue culture and bioprocessing technologies have been developed. These are based on the model that phenolic metabolites in plants are efficiently produced through an alternative mode of metabolism linking proline synthesis with the pentose phosphate pathway. Proline biosynthesis coupled to the pentose phosphate pathway stimulates the synthesis of NADPH(2) and sugar phosphates for anabolic pathways, including phenolic and antioxidant response pathways. The reducing equivalents for mitochondrial oxidative phosphorylation are provided by proline replacing NADH, with oxygen being the terminal electron acceptor. Using this system, techniques have been developed to isolate high phenolic clonal lines of food-grade plants from single heterozygous seeds. Applying the same model, elicitation concepts and techniques have been used to over-produce phenolic metabolites in seeds and sprouts. In both clonal and seed sprout systems, exogenous treatment of phenolic phytochemicals from a non-target species elicited endogenous stimulation of phenolic synthesis and potentially an antioxidant response. From these initial plant antioxidant response investigations. a model has been proposed in which the proline-linked pentose phosphate pathway is critical for modulating protective antioxidant response pathways in diverse biological systems, including humans. The proposed proline-linked pentose phosphate pathway model. when confirmed precisely, provides a mechanism for understanding the mode of action of phenolic phytochemicals in modulating antioxidant pathways in relation to human health. This can provide dietary and nutritional mechanisms as well as new strategies to manage the oxidation-linked diseases through improvement of host physiological response. In other environmental applications, this model can be used to screen and design plants targeted for phytoremediation of aromatic pollutants and adaptation of plants in various stressed environments, including outdoor adaptation of tissue culture and transplanted seedlings for better food production. (C) 2003 Elsevier Ltd. All rights reserved.